An easy way to do it is select the thing you want to gray, and go to image->Hue/Saturation ( Ctrl+U ), and press colorize. Drop the Saturation to 0 and move the lighting to level of gray.
couple of different ways to do this. The easiest way is to just hit CTRL+SHIFT+U, which will desaturate the image. If you never want color in the image you can go to image-mode-greyscale. The hue/saturation method mentioned above works well, as it gives you a certain level of control, but for the best control create a 'channel mixer' adjustment layer, and check the 'monochrome' box in the bottom left. Then adjust the RGB sliders to your liking.